This invention relates to a water remoistenable adhesive coated product which has minimum curl over a wide range of humidity conditions and takes the form of adhesive tapes or labels.
Water remoistenable adhesive coated papers are well known in the art. They are commonly referred to as gummed labels or tapes. These gummed products are commonly obtained by preparing a solution of a water soluble gum such as animal glue or dextrin, applying a thin film thereof to paper, and evaporating the solvent to give a dry, non-tacky, potentially adhesive material on paper or other substrates.
Practical applications for adhesive coated papers and products manufactured in this manner are limited by the tendency of the gummed sheets or tapes to curl or roll up. Not only does this become a problem in the application of the adhesive product to a desired substrate, but printers cannot properly run these adhesive coated papers in their presses. The curl effect is due to the differing response of the adhesive coating and the paper backing on exposure to changing humidity conditions. Both the paper backing and the adhesive layer expand when they absorb moisture from the atmosphere and contract as they release moisture to the atmosphere. However, the degree of expansion and contraction is generally greater in the adhesive coating than in the paper backing. Thus, when a sheet is exposed to an atmosphere of high moisture content, the adhesive layer will expand more than the paper backing, causing the sheet to curl away from the adhesive side. If the same sheet is exposed to an atmosphere of low moisture content, the adhesive layer will contract more than the paper backing and the sheet will curl towards the adhesive side.
The most important and effective method used by manufacturers in an attempt to overcome the curl problem in producing what the trade refers to as gummed flats is to make the adhesive material in the adhesive coating discontinuous in some manner. Both mechanical and coating formulation approaches have been used with varying degrees of success.
The patent art lists many mechanical approaches for making a flat gummed product. McLaurin, U.S. Pat. No. 890,570 and Gautier, U.S. Pat. No. 2,275,059 roughened the surface of gummed stock by embossing methods. Knappstein, U.S. Pat. No. 1,148,783 ruptured the coating by incised engraving. Holt, U.S. Pat. No. 1,185,600 developed a breaker machine and method for breaking the adhesive coating on gummed paper. McLaurin, U.S. Pat. No. 1,619,027 applied powdered gum to a wet web to give a discontinuous coating. Humphner, U.S. Pat. No. 2,400,995 perforated the paper.
Formulation and coating application techniques have also been used to obtain a discontinuous particle coating effect. Schroder, U.S. Pat. No. 1,297,406 applied glue powder to a solvent tackified size coating. Noble, U.S. Pat. No. 1,361,894 dusted powdered glue or dextrin onto a rubber cement coating. Holt, U.S. Pat. No. 3,090,353 developed a process for electrostatic gumming. The product is obtained by electrostatic deposition of an adhesive onto a thermoplastic binder and the coating is fixed by heat fusion. Solvent dispersions of powdered gums in solutions of resins in organic solvents have been used by Davis, U.S. Pat. No. 2,793,966; Prior, U.S. Pat. No. 3,104,179; Neumann, U.S. Pat. No. 2,477,344 and Russo, U.S. Pat. No. 2,978,343.
One industrial process obtains the adhesive product by dispersion of a water remoistenable gum powder in a toluene solution of a water-sensitive binder such as modified polyvinyl acetate or polyvinyl pyrrolidone.
A second manufacturer electrostatically deposits powdered gum adhesive on a modified polyvinyl acetate coating. The sheet is then heated to soften the binder and allow powdered gum to penetrate into the coating.
A third process obtains gummed flats by coating on a substrate a water-in-oil type emulsion of an aqueous solution of the adhesive gum emulsified in a solvent solution of a modified polyvinyl acetate. On drying, adhesive particles are dispersed in a continuous film of polyvinyl acetate.
Neither the mechanically treated coated adhesive product nor the dispersions of powdered gums in a resin binder give a highly satisfactory flat gummed adhesive. The adhesive coatings which have been mechanically fractured are re-sealed into a continuous film by continued exposure to atmospheric moisture and the curling problem is again evident. With respect to the products obtained from a dispersion of the powdered gum in a resin binder, the binder acts as a diluent and contaminant for the gummed adhesive. The binder coated glue particles require more time for water to penetrate and activate the glue than is required for the unmodified adhesive. The polyvinyl acetate binder reduces the adhesive properties of the adhesive coating and does not stick as well as unmodified adhesive coatings. The coating weight of the adhesive layer must be increased to contain as much effective adhesive as is contained in an unmodified adhesive coating. This results in higher materials costs as well as a reduction in coating production speed.